(1) Field of the Invention
The invention relates to a method of photolithographic etching of metal lines, and more particularly, to a method of photolithographic etching of sub-quarter micron metal lines without undercutting in the manufacture of integrated circuits.
(2) Description of the Prior Art
It is desired for metal lines to have a vertical profile. This is not always easy to achieve, especially for sub-quarter micron metal lines where the photoresist mask must become thinner to improve lithographic resolution. Because of poor photoresist selectivity, especially with a deep ultraviolet (DUV) light source, metal etching using a hard mask; i.e. an oxide mask; will be the trend for sub-quarter micron logic technology. Thus, metal sidewall. protection becomes even more important than previously.
FIG. 1 illustrates in cross-sectional representation a partially completed integrated circuit device of the prior art. Semiconductor substrate 40 contains semiconductor device structures, not shown. A metal line stack is shown on the substrate. Barrier metal layer 44 (for example, titanium/titanium nitride) is on the bottom of the stack. The metal layer 46, such as AlCu, overlies the barrier layer. An antireflective coating (ARC) 48 is at the top of the stack. Silicon oxide hard mask 50 has been formed over the ARC layer using the pattern of the photoresist mask 52.
Conventionally, the metal lines are etched using the etchant gases BCl.sub.3, Cl.sub.2, and N.sub.2. Etching continues until the time that the mean metal stack thickness is etched through. Then, because of thickness and etch rate non-uniformities in all etching processes, the metal stack is overetched for a time sufficient to completely etch through all of the desired material. In addition, the barrier metal layer is etched through in the overetching step. It has been found that the metal sidewalls will not be attacked if BCl.sub.3 is not used during this overetching. However, an undercut 54 will be found near the ARC interface with the metal layer 24, as shown in FIG. 2. Cl.sub.2 will attack the metal line near the interface at the end of the etching time. The N.sub.2 chemical passivation (C--N bond) cannot prevent the Cl.sub.2 erosion effectively, but only smooths the metal sidewall.
U.S. Pat. No. 4,855,016 to Jucha et al teaches etching tungsten with SF.sub.6 alone or in combination with a bromine source and teaches etching aluminum using BCl.sub.3 and Cl.sub.2 gases. U.S. Pat. No. 5,126,008 to Levy teaches etching aluminum using SF.sub.6 and bromine. U.S. Pat. No. 4,214,946 to Forget et al teaches using Cl.sub.2 and SF.sub.6 to etch silicon. U.S. Pat. No. 4,741,799 to Chen et al etches silicon using SF.sub.6 and a hydrocarbon. U.S. Pat. Nos. 5,217,570 and 5,540,812 to Kadomura teach etching of a barrier layer using S.sub.2 F.sub.2 to form a sulfur-based sidewall protection for a metal layer. U.S. Pat. No. 5,326,427 to Jerbic teaches using atomic chlorine and atomic fluorine to etch titanium, but not aluminum. Co-pending U.S. patent application Ser. No. 08/998,673 (TSMC-97-210) to Shue et al teaches using a fluorine-doped silicate glass hard mask to prevent undercutting of metal lines during etching,